This invention relates to a mixing head for mixing at least two components which react together, preferably to form a foam, comprising a housing in which is arranged a guide bore for an ejector piston, one section of the bore forming a mixing chamber into which inlet apertures open for the components, one end of the mixing chamber being bounded by the end face of the ejector piston when the piston is withdrawn, the outlet aperture being arranged at the opposite end of the mixing chamber.
Mixing heads of this kind generally operate on the counterflow injection principle, i.e., the injection apertures, which are in the form of nozzles or perforated plates, are situated opposite one another.
To achieve thorough mixing, a certain pressure must be maintained within the mixing chamber. This pressure is difficult to measure because of the small volume of the mixing chamber. It is therefore generally based on the theoretical velocity of the reaction mixture leaving the mixing chamber. It has been found, for example, that for the production of foams based on polyurethane, a sufficient intensity of mixing is ensured if the mixture is discharged at the rate of 20 to 40 meters per second. In the known mixing heads, pressure in the mixing chamber is produced by means of swirl elements or perforated shields placed in the outlet element behind the mixing chamber.
Since these mixing heads are generally used for filling molds with reaction mixture, it is necessary to ensure that the mixture leaves the mixing chamber in a quiet, steady flow so that it enters the cavity of the mold in a laminar stream. The purpose of this requirement is to obtain a closed flow front and ensure that no air bubbles are beaten into the mixture as these could give rise to faults in the finished molded product. This laminar flow is obtained generally by means of deflecting and calming zones arranged in the outlet elements. Such mixing heads with fittings installed in the outflow elements must, however, be flushed with solvent or compressed air. The residues of reaction mixture thereby removed as waste, pollute the surroundings.
New, positively controlled mixing heads advantageously operate without the use of rinsing fluids since the residues of reaction mixture left in the mixing chamber are ejected by means of an ejector piston towards the end of the mold-filling operation. In this case, it is just as impossible to adapt the pressure in the mixing chamber to the prevailing requirements as it is to ensure a laminar flow from the mixing chamber into the mold cavity.
Since mixing cannot be achieved with the required intensity, attempts have also been made to provide aftermixers between the mixing chamber and the cavity of the mold. These devices do improve mixing and calm the turbulent stream of mixture leaving the mixing chamber. Both the aftermixers and the sprue channel are placed in the plane of separation of the mold. When the product has hardened, the mixture left in the sprue channel and aftermixer must be removed from the mold as waste. The sprue mark left on the molded article is a blemish and must be removed.
Another design of mixing head has an ejector piston which can be pushed forwards as far as the outlet aperture of the mixing chamber towards the end of the mold filling process, and this outlet aperture is directly adjacent to the mixing chamber. It is therefore possible to produce moldings free from sprue marks. Due to the inefficient mixing process, however, the products have defects in the form of streaks and bubbles.
It is therefore an object of the present invention to provide a mixing head which ensures sufficiently vigorous mixing by virtue of the fact that the pressure in the mixing head is adjustable, and which can be operated without the use of rinsing fluid and enables the mixture to enter the mold cavity as a laminar stream and ensures production free from waste.